The present invention relates to controllable inductive devices. More particularly, the invention relates to controllable transformers.
A transformer comprising orthogonal windings is previously known from U.S. Pat. No. 4,210,859, to Meretsky et al. of Apr. 18, 1978 (hereinafter xe2x80x9cMeretskyxe2x80x9d). However, the known solution manifests several disadvantages. Some of these disadvantages are described below.
In general, the problem with the prior art as illustrated by Meretsky is that it does not present a complete picture of how the manipulation of the domains with a DC control current affects the magnetisation in relation to the connection between two orthogonal windings. In Meretsky, a device is described which is developed on the basis of a test conducted on a ferrite pot core with dimensions 18xc3x9711 mm, and with current levels in the mA range. Ferrite, however, is not suitable for use at high power levels, for example, because of the high material costs associated with it. The high costs limit the size of a ferrite core from the production engineering point of view. Further, higher power levels can be transferred by increasing the frequency of the voltage that has to be converted, but this requires complicated and expensive power electronics.
Meretsky illustrates a connection diagram for a variable transformer solution with 4 windings: a primary main winding, a secondary main winding arranged at a right angle to the primary winding, and two control windings, one for each main winding. The mode of operation is such that a variable DC current in both control windings will result in a transfer of AC voltage from the primary winding to the secondary winding. A transformer of this kind cannot be considered a realistic option, particularly if it is to be applied outside the mA range, because a DC current in the control windings will rotate the domains in the magnetic material in an unfavourable direction for connection in one half cycle of the primary voltage. These domain rotations cause harmonics in the secondary voltage. This phenomenon, is not taken into consideration in Meretsky.
In order to be able to implement a realistic solution for a variable power transformer, the problem arises that the control winding on the primary side is transformatively connected to the primary winding and will be under voltage from the primary side, thereby making it very difficult to regulate without extensive filtering.
Meretsky also discloses a transformer connection (FIG. 18) where windings with right-angled axes are interconnected in series two by two. The publication states that the core""s utilisation can be increased by using such a connection. This is not correct, however, since the magnetic fields for the windings are summed vectorially and the described effect will not be achieved.
Meretsky also describes (FIG. 20) a variable delay between the input and output voltage in a case where the control windings each carry current and are interconnected in series. Phase distortion is involved here since the fields through the primary and the secondary winding are shifted via the domain directions. With the control windings connected in this manner, the device will not work for a power transformer used as a phase inverter, since the connection from the primary winding will influence the control current to such an extent that in principle the same connection as mentioned earlier (FIG. 18) will be obtained.
The present invention addresses the shortcomings of the prior art by implementing a transformer in which the domain rotation is controlled.
In one aspect of the invention, a magnetisation in a transformer core provides a connection from a primary side to a secondary side by means of a current in a control winding. As a result of the orientation of a primary winding, a secondary winding and the control winding, two magnetisation currents, which are orthogonal, are summed in such a manner that the domain direction is changed linearly in a direction that is at an angle to the secondary winding. Further, an induced voltage in the secondary winding will be dependent on the size of this angle.
In one embodiment, the magnetisation of the transformer is controlled by means of a pulsed DC or a pulsed AC control current in the control winding which is located orthogonal to the primary control winding. The direction of the domains can be held constant as a result of the controlled magnetisation. The domain control also can be used to avoid a simultaneous change of the domain direction and the field strength of magnetisation. In a version of this embodiment, a constant domain direction is achieved by means of accurate dosing of the control current in relation to the primary winding""s magnetisation current and the ampere-turn balance with the secondary winding.
In a further embodiment of the invention, a core plate is used which has special properties with regard to permeability. In a version of this embodiment, a laminar material is used where the magnetisation curve is the same for all directions in the plate. This involves the use of non-directional plate. However, in yet another embodiment of the invention, a directionally oriented plate is used.
The invention can also be implemented in a variable transformer/frequency converter device comprising a body of a magnetic material, a primary winding (or first main winding) wound round the body about a first axis, a secondary winding (or third main winding) wound round the body about a second axis at right angles to the first axis, and a control winding (or second main winding) wound around the body about a third axis, coincident with the second axis.
In another aspect, the invention concerns a method for controllable conversion of a primary alternating electrical signal to a secondary alternating electrical signal by the use of a device comprising a body of a magnetic material, a primary winding (or first main winding) wound round the body about a first axis, a secondary winding (or third main winding) wound round the body about a second axis at right angles to the first axis, and a control Winding (or second main winding) wound around the body about a third axis, coincident with the second axis. In one embodiment, the primary winding is supplied with a primary alternating electrical signal, the control winding is supplied with an alternating voltage which is either in phase or shifted by 180xc2x0 relative to the primary alternating electrical signal, and the control winding is supplied with a variable current. As a result the transformer""s conversion ratio is controlled by means of the variable current.
In a further embodiment, an amplitude adjustment of the alternating voltage changes at least one of domain directions in the magnetic material and a magnetisation angle between the primary winding and the secondary winding. An inductance is introduced in the control circuit, an electromagnetic force from the secondary winding is added to an electromagnetic force from the control winding, and a phase angle rotation between the primary winding and the secondary winding is compensated. This embodiment results in a change in the voltage transfer of the transformer and a phase angle rotation that varies according to load conditions. Additionally, the magnetisation angle between the primary winding and the secondary winding is influenced by the added electromagnetic force. Also, the effect of a direct transformative connection between the secondary winding and the control winding is suppressed. A resulting controlled transformation effect is achieved by obtaining a primary winding response to a load change in a secondary load.
In still another embodiment, the transformer device includes a hollow magnetisable core with an internal winding compartment for internal windings and an external winding compartment for external windings. In a version of this embodiment, the transformer device includes three windings: a primary winding located in the external winding compartment; an associated control winding located in the internal winding compartment; and a secondary winding located in the internal winding compartment. The windings in the external winding compartment and the windings in the internal winding compartment are aligned at right angles (perpendicular) to each other. As a result, orthogonal magnetic fields are created. Alternatively, in yet another embodiment, the internal winding compartment may house both the primary winding and the external winding compartment may house the secondary winding and the control winding. The transformer device can be used in a frequency converter. In a version of this embodiment, the frequency converter is used in the MVA range.
According to an embodiment of the invention, a magnetisation current is established in the control winding that conforms to the magnetisation current from the primary winding in amplitude in order to enable a transformative connection to be established between the primary and secondary winding that does not produce undesirable frequencies in the secondary voltage. Without this magnetisation, the desired transformative connection to the secondary winding will not result. However, there will be some degree of connection on account of the winding""s extension in the compartment which provides one induced component. Another induced component will result from nonlinearities in the material.
A control voltage, in a method according to an embodiment the invention, is in phase or antiphase with the primary voltage in order to achieve a distortion-free transformative connection. Through a slow change in the amplitude of the control voltage, the direction of the domain change or the magnetisation angle between the primary winding and secondary winding can be changed. The change allows the voltage transfer to be controlled. Through introduction of an inductance in the control circuit it is possible to suppress the effect of the direct transformative connection between the secondary winding and the control winding. The secondary winding will act as a control winding, with its electromotive force (mmf) being added to electromotive force (mmf) from the control winding to influence the magnetisation angle between the primary winding and the secondary winding. Basically, it is not possible to isolate this effect from the secondary winding and we shall obtain a variable phase angle rotation between primary and secondary according to the load conditions. However, we can compensate for this by using a phase compensation device as described in PCT/NO01/00217 to compensate for the phase angle rotation. Because the primary winding will immediately respond to any load change from the secondary side, according to Lenz""s law we shall achieve the desired regulating transformer effect.
The transformer according to one embodiment of the invention, includes only one control winding located in the winding compartment together with the secondary winding. In principle, a control winding in the primary winding compartment is not necessary because the primary winding will rotate the domains in its direction and also rotate any domains established from a current in the secondary winding in the same direction. In order to obtain transformative connection between the orthogonal windings, the domains must be rotated as mentioned above in order to efficiently produce a magnetisation that is in a favourable direction for transformative connection between the primary and the secondary winding. The rotation may also be described as xe2x80x9ctwistingxe2x80x9d the secondary winding relative to the primary winding so that some of the field from the primary winding passes through the secondary winding.
In order to achieve transformer effect without distortion of the primary voltage, according to an embodiment of the invention, an (AC) alternating voltage is used on the control winding, which as previously mentioned is located in the same winding compartment as the secondary winding. When current begins to flow in the control winding, this current will reinforce the connection with the primary side because the field from the secondary current and the field from the control current help rotate the domains in the correct direction.
In another embodiment, the control voltage in the transformer will be in phase with or phase shifted 180 degrees relative to the voltage on the primary side in order to obtain a distortion-free transformation. The current in the control winding can be regulated by a system that monitors the primary and the secondary current and/or voltage as well as the control current, thus enabling the transformative connection and allowing the electrical angle between the windings to be controlled by means of the alignment of the domains. As mentioned before, the values of current and voltage in each of the primary winding, the secondary winding, and the control winding will give a clear indication of the state of the domains (rotation and magnetisation). Thus, these parameters together with reference values can be used for controlling the transformer""s operation and response to different operation conditions.
In one embodiment, domains of a magnetisable core of a transformer according to an embodiment of the invention are aligned by energizing the first winding, monitoring a current in the first winding, monitoring a current in the second winding, and exciting the third winding to compensate for domain changes established by the second winding.
In another embodiment, a method of controlling the orientation of a field in a transformer includes generating a primary field in a first direction, generating a secondary field in second direction orthogonal to the first direction, generating a control field in a third direction which is coincident to the first direction, and adjusting the control field to control a direction of the primary field.
The transformer, according to an embodiment of the invention, may also advantageously be employed as a controlled rectifier or frequency converter. In order to achieve such a controlled rectifier effect from this transformer, at least two methods may be employed.
For example, in one method according to an embodiment of the invention, the primary winding of a first controllable transformer to is connected to a power supply. A central point of the secondary winding of the first transformer is connected to a load. The ends of the first secondary winding are connected to a first diode rectifier topology. An AC voltage is supplied to the first control winding in the first transformer. The primary winding of a second controllable transformer is connected to a power supply. A central point of the secondary winding of the second transformer is connected to the load in parallel with the central point of the first secondary winding. The ends of the secondary winding of the second transformer are connected to a second diode rectifier topology, and an AC voltage is supplied to the second control winding in the second transformer. In one version of this embodiment, a frequency converter for motor control is provided.
In yet another method according to an embodiment of the invention, a frequency controlled output is provided to a load. According to this embodiment, during a first period, a primary winding of a first transformer is energized, a primary winding of a second transformer is energized, a control winding of the first transformer is energized, the second transformer is maintained in an off state, and a rectified output of a secondary winding of the first transformer is supplied to the load. During a second period, the control winding of the first transformer is de-energized, a control winding of the second transformer is energized, and the rectified output of a secondary winding of the second transformer is supplied to the load. Further, during the first period the rectified output of the first transformer is a positive voltage, during the second period the rectified output of the second transformer is negative voltage, and the frequency controlled output is varied by controlling a length of the first period and a length of the second period.
In still another method according to an embodiment of the invention, rectifying is implemented by supplying an alternating voltage from a power supply to a first transformer and a second transformer, a secondary winding of the first transformer is connected to a load, and a secondary winding of the second transformer is connected to the load in parallel with the secondary winding of the first transformer. Further, at a first zero crossing of the alternating voltage, a first pulsed control voltage is supplied to a control winding of the first transformer where the first pulsed control voltage includes a signal that is both in-phase and of opposite polarity relative to the alternating voltage. At a second zero crossing of the alternating voltage, a second pulsed control voltage is supplied to a control winding of the second transformer where the second pulsed control voltage includes a signal that is both in phase and of an opposite polarity relative to the alternating voltage. Additionally, the first transformer has a primary winding connection comprising a first end, the second transformer has a primary winding connection comprising a second end, and the first end and the second end are connected to a common terminal of the power supply.
The invention is a further development of the device set forth in PCT/NO01/00217, the entire contents of which are incorporated herein by reference. However, the invention relates to a new device, since the primary and the secondary windings do not have parallel, but right-angled winding axes, and a control of the domain state is included in the present invention.
The invention will now be described in detail with reference to the drawings.